It has been known to prepare printed circuits, chemically-machined parts, etched articles and the like by first applying a thin layer of a photosensitive resist to the surface of a substrate. There are two types of resists available: negative-working and positive-working. A negative-working resist is initially soluble in its developer but, after exposure to light of the proper wave length, becomes hardened and insoluble. Positive-acting resists are initially insoluble but become soluble upon being exposed. Polymeric images are then formed by exposing the layer, imagewise, to actinic radiation. Exposed or unexposed areas of the layer, as the case may be, are removed to form a resist image of polymeric material. The areas left unprotected by the resist image are permanently modified by being etched or having material deposited thereon. Finally, the remaining resist is stripped from the part.
U.S. Pat. No. 3,887,450 (Giliano et al.) describes using alkaline aqueous solutions as developers for negative-working photoresists. Among a large group of suggested bases, tetrasubstituted ammonium hydroxide and basic soluble salts thereof are disclosed. Strong alkalies or proprietary stripping formulas are taught for stripping.
U.S. Pat. No. 4,379,830 (Deutsch et al.) describes an improved alkaline developer for positive-working photoresists employing bases made from, among a list of anions, phosphates, and among a list of cations, quaternary ammonium ions, together with at least one neutral salt.
U.S. Pat. No. 4,530,895 (Simon et al.) describes an alkaline developer for positive-working photoresists employing quaternary ammonium bases and salts thereof.
U.S. Pat. Nos. 4,294,911 and 4,464,461 (Guild) describe the use of an aqueous solution of quaternary alkanol ammonium hydroxide as a developing agent for positive-working photoresists and positive-working lithographic printing plates. They teach that these quaternary ammonium compounds decompose without stabilizers.
U.S. Pat. Nos. 4,239,661 and 4,339,340 (Muraoka et al.) describe the use of an aqueous solution of trialkyl(hydroxyalkyl) ammonium hydroxide, preferably choline base, as a cleaning agent for semiconductor wafers, as an etchant for metal layers used as wiring, and as a developer and stripper for positive working photoresist films.
Choline, also known as choline base, (2-hydroxyethyl)trimethylammonium hydroxide, is a well-known organic base suitable for stripping photoresist and for a variety of other uses. Aqueous and lower-alcohol solutions of choline base are useful in electronic applications such as photoresist developing and stripping, anisotropic etching, and cleaning.
Aqueous solutions typically are about 0.01 to 40 weight percent (wt.%) choline base. They are generally prepared and shipped as concentrates having at least 10 wt.% choline base. The concentrates are typically diluted to concentrations of about 1 to 5 wt.% choline base for use.
Solutions of choline base in lower alcohols such as methanol, in concentrations as high as 50 wt.% choline, have been suggested for use as replacements for (alcoholic) solutions of common alkalis, catalysts, curing agents, hydrolytic agents, neutralizing agents and solubilizing agents, as well as for pH control.
It has been hypothesized that when choline base contacts polymeric photoresists it partially decomposes into trimethylamine (TMA) and ethylene glycol and its oxidized decomposition products, creating adequate action to make it an effective stripping agent. Unfortunately, this results in extremely repugnant fishy odors, probably due to the TMA being released.
While some decomposition may be desired when the choline base contacts the polymeric photoresist, choline base's tendency to decompose, particularly during shipment and storage, has the unwanted consequences of darkening and changing its performance as a developing and stripping agent. To prevent or retard such decomposition with its consequences, inexpensive stabilizers that do not interfere with the intended use have been sought.
U.S. Pat. No. 4,686,002 teaches one such system for solutions of choline base in water and/or lower alcohols. Formaldehyde and paraformaldehyde are taught as improved stabilizers over the prior art stabilizers of sulfites and semicarbazides used in aqueous choline base and of the ethylenediamine, typically used in methanolic choline base.
But, stabilizers add to the cost of operation, only reduce the rate of decomposition and may be the source of unwanted ions that can have a deleterious effect on the electronic product being treated with the choline base. Tight limits for particular ions and residues that may be present are specified by the electronics industry. For example, in printed wire board manufacture, no residue following the normal post-bake period is allowed because even traces of impurities such as alkali metals would interfere in the operation of the electronic circuits.
It would be desirable to have a developer/stripper as effective as choline base, but which is stable without the addition of stabilizers that add cost, are only partially effective and impart undesirable ions or leave residues on the electronics workpiece and which doesn't have an extremely repugnant fishy odor.